1. Field of the Invention
The invention relates to a method for the continuous psychrometric measurement of the concentration of a vaporous component in a gas, in particular the measurement of the moisture content of air.
1. The Prior Art
Psychrometric measurement of the concentration of a vaporous component in a gas takes place by causing the gas to flow past a "dry" thermosensor measuring the temperature of the gas flow, and a "wet" thermosensor, the surface of which is kept completely moistened with liquid of the same kind as the vaporous component. If the gas is unsaturated with the vaporous component, a vaporization of liquid will take place from the surface of the wet thermosensor, whereby, in the ideal case of a psychrometric measurement, the temperature of the wet thermosensor will drop until the supply of heat per time unit by convection from the flow of air is exactly equal to the energy consumption per time unit for the vaporization of liquid from the moistened surface. If the gas is saturated with the vaporous component, no vaporization takes place from the moistened surface, and the wet thermosensor assumes exactly the same temperature as the dry thermosensor.
Measurement of the temperature of the air by means of the dry thermosensor, measurement of the temperature of the wet thermosensor, and measurement of the pressure of the air will suffice for calculating the moisture content of the air, and the same of course applies to other gases and vaporous components.
One of the difficulties in obtaining ideal conditions for the psychrometric measurement is, however, that heat may be conveyed to or detracted from the moistened surface of the wet thermosensor by the liquid used for moistening this thermosensor, so that its temperature will assume a value higher or lower, respectively, than that at which dynamic equilibrium exists between the heat convection from the gas to the moistened surface and the heat consumption for vaporization from that surface. To limit this source of error it is necessary in the case of the known psychrometers to keep the amount of liquid per time unit used for the moistening as small as possible, but still sufficient for keeping the wet thermosensor completely moistened. In the known psychrometric methods the total quantity of liquid supplied to the wet thermosensor is normally vaporized. Impurities in the supply liquid and dust, salt and oil drops precipitated from the gas flow will therefore be accumulated in the liquid film covering the wet thermosensor whereby the vapor pressure of the liquid will be reduced, and it becomes difficult to moisten the thermosensor efficiently, whereby measuring errors will occur. It is therefore necessary in the known methods to clean the wet thermosensor frequently.
The required rate of liquid supply to the wet thermosensor depends strongly on the degree of saturation of the gas in respect of the vaporous component in question. Therefore, in carrying out the known methods it is difficult to avoid a partial drying-up of the wet thermosensor at low degrees of saturation without at the same time invalidating the measurement at high degrees of saturation as a consequence of a too high rate of liquid supply.
The last-mentioned difficulties have been avoided in a psychrometric measuring apparatus disclosed in U.S. Pat. No. 3,157,049. In this apparatus the wet thermosensor is arranged at the bottom of an open flat dish, in which a thin liquid layer is present, the level of which is maintained constant by the supply of liquid through a level control device. Thus, in this apparatus an amount of liquid is at any time supplied to the wet thermosensor which is exactly the same as the amount removed by vaporization. However, it is not avoided that heat can be conveyed to or detracted from the wet thermosensor by the liquid supplied thereto, whereby measuring errors can occur as above described, particularly in the case of low degree of saturation of the gas in respect of the vaporous component, and it is not avoided either that impurities in the supply liquid and dust, salt, oil drops and the like precipitated from the gas flow will be accumulated in the layer of liquid covering the wet thermosensor so that a careful watch and frequent cleaning of the container will be required to obtain security against measuring errors resulting from pollution.